As an approach to measuring birefringence in a birefringent medium, the crossed-Nicol method is well known. In this approach, a combination of a polarizer and an analyzer, which are perpendicular to each other, and a birefringent medium, which is disposed therebetween as a measurement target, are rotated relative to each other, and during the rotation, the intensity Iout(θ) of light transmitted through the polarizer, the measurement target, and the analyzer is measured, and birefringence Δn in the measurement target is determined by the following equation.
                                          I            out                    ⁡                      (            θ            )                          =                              I            in                    ⁢                                    sin              2                        ⁡                          (                                                π                  λ                                ⁢                Δ                ⁢                                                                  ⁢                nd                            )                                ⁢                                    sin              2                        ⁡                          (                              2                ⁢                θ                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]            Here, Iin is the intensity of light incident from the polarizer, θ is the relative rotation angle of the measurement target, and d is the thickness of the measurement target. Moreover, Δnd, which is represented by the product of the birefringence Δn and the thickness d, is the optical path difference between extraordinary and ordinary components of light with a wavelength λ passing through the measurement target, and the optical path difference causes a phase difference δ.
                    δ        =                                            2              ⁢              π                        λ                    ⁢          Δ          ⁢                                          ⁢          nd                                    [                  Equation          ⁢                                          ⁢          2                ]            In this manner, the birefringence Δn is derived from the phase difference δ of the light having passed through the measurement target with the thickness d, and therefore, birefringence measurement is synonymous with phase difference measurement, and in some cases, might be referred to as birefringent phase difference measurement.
However, this approach requires the combination of the polarizer and the analyzer and the measurement target to be rotated at least 180° relative to each other, resulting in issues of time-consuming measurement and necessity for a extensive rotating mechanism. Accordingly, there has been proposed a rotating analyzer method in which the polarizer creates circularly polarized light to be incident on the measurement target and only the analyzer at the end is rotated, but such a method still requires a rotating mechanism.
To overcome the issues, there have been proposed various approaches which require no rotating mechanism. For example, Patent Document 1 proposes a birefringence measurement device 100 (see FIG. 15) including a means for irradiating a measurement target 20 with polarized light L10, beam splitters 101 and 102 for dividing polarized light L11 transmitted through the measurement target 20 in three components, analyzers 103, 104, and 105 for allowing the three components of the divided polarized light L11, which oscillate in specific directions, to pass therethrough, optical detectors 106, 107, and 108 for measuring the intensities of the light transmitted through the analyzers 103, 104, and 105, and an arithmetic device 109, such as a computer, for determining an elliptical trajectory of the polarized light L11 on the basis of the results obtained by the optical detectors 106, 107, and 108. In the birefringence measurement device 100, the analyzers 103 and 104 differ in angle by 45°, and the analyzers 103 and 105 differ in angle by 90°.
The birefringence measurement device 100 makes it possible to determine birefringence Δn in the measurement target 20 on the basis of the relationship between a known polarization state of the polarized light L0 and a polarization state of the polarized light L11 determined by the arithmetic device 109.
Furthermore, Patent Document 2 proposes a birefringence measurement device 200 (see FIG. 16) in which a measurement target 20 is irradiated with light flux having a known polarization state (e.g., circularly polarized light L20) and a polarization state of transmitted light L21 is detected by a polarizer array 201 and an area sensor 202 (e.g., a CMOS camera). As shown in (B) of FIG. 16, the polarizer array 201 includes a plurality of polarizer units 203 in series both in X and Y directions, and each polarizer unit 203 includes 4×4 (=16) polarizers different in transmission axis from one another.
In the birefringence measurement device 200, when compared to the birefringence measurement device 100, the polarizer array 201 plays the same role as the analyzers 103, 104, and 105, and the area sensor 202 plays the same role as the optical detectors 106, 107, and 108. Moreover, the birefringence measurement device 200 does not require the beam splitters 101 and 102 as does the birefringence measurement device 100. Accordingly, the birefringence measurement device 200 makes it possible to measure a two-dimensional distribution of birefringence Δn in the measurement target 20 by a simpler configuration than the configuration of the birefringence measurement device 100.